1. Technical Field
The present invention relates to the technical field of amplifying apparatuses in which the power consumption can be reduced.
2. Background Art
WO00/72546 discloses a technique in which, in an amplifying apparatus that operates with two kinds of high and low power supply voltages from each of power supplies respectively having positive and negative polarities, the power supply voltages are controlled in accordance with the amplitude of a signal.
FIG. 11 shows an example of an amplifying apparatus for adjusting power supply voltages in accordance with the amplitude of a signal. In the example, high power supply voltages are used at ±2 V, and low power supply voltages for improving the output efficiency are used at ±1 V which are a half of the high power supply voltages of ±2 V. The configuration is employed because, owing to the property of a so-called charge pump, the two-division structure causes the circuit operation to be mostly simplified, and the output loss to be reduced.
In the conventional technique, when the low power supply voltages of ±1 V are used, the gate-source voltages Vgs of output transistors are 2 V at the maximum. In the case where the load impedance is low, i.e., for example, 8 or 16Ω and the gate-source voltage Vgs is small, it is difficult to obtain excellent linearity, and hence the saturation voltages of the output transistors become high.
Therefore, an output voltage which can be output by the low-voltage operation is smaller in amplitude than the power supply voltages of ±1 V, and there arises a technical problem in that the improvement of the power efficiency is impeded.